Coordination and Response

The One Thing

Coordination and response enable organisms to detect changes, process information, and produce suitable responses. Nervous control provides rapid, precise responses, whereas hormonal control and plant growth responses generally act more slowly and last longer.

Reactions, Processes and Experiments

Key Terms

• Stimulus: A change in the internal or external environment that can be detected and may cause a response.
• Receptor: A cell or group of cells that detects a particular stimulus.
• Sense organ: An organ containing receptor cells that detect stimuli, such as the eye, ear, skin, nose, and tongue.
• Central nervous system: The brain and spinal cord, which receive information, process it, and coordinate responses.
• Peripheral nervous system: All nerves outside the brain and spinal cord that carry impulses to and from the central nervous system.
• Neurone: A nerve cell specialized to carry electrical impulses.
• Sensory neurone: A neurone that carries impulses from a receptor to the central nervous system.
• Relay neurone: A neurone in the central nervous system that passes impulses between sensory and motor neurones.
• Motor neurone: A neurone that carries impulses from the central nervous system to an effector.
• Effector: A muscle or gland that carries out a response.
• Nerve impulse: An electrical signal that travels along a neurone.
• Synapse: A tiny gap between neurones where chemicals carry the signal from one neurone to the next.
• Reflex action: A rapid, automatic response that usually protects the body from harm.
• Reflex arc: The pathway followed by an impulse during a reflex action: receptor, sensory neurone, relay neurone, motor neurone, and effector.
• Hormone: A chemical messenger produced by an endocrine gland and transported in the blood to a target organ.
• Endocrine gland: A gland that releases hormones directly into the bloodstream.
• Target organ: An organ or tissue with receptors for a particular hormone.
• Adrenaline: A hormone released by the adrenal glands during danger or stress; it increases heart rate, breathing rate, and blood glucose availability.
• Insulin: A hormone produced by the pancreas that lowers blood glucose concentration by increasing glucose uptake and conversion to glycogen.
• Glucagon: A hormone produced by the pancreas that raises blood glucose concentration by causing glycogen to be converted to glucose.
• Homeostasis: The maintenance of a stable internal environment despite changes inside or outside the organism.
• Negative feedback: A control mechanism in which a change triggers responses that reverse or reduce the original change.
• Eye: A sense organ containing light receptors in the retina; the iris controls pupil size and the lens focuses light onto the retina.
• Accommodation: The change in lens shape that allows the eye to focus on near and distant objects.
• Ear: A sense organ for hearing and balance; sound vibrations stimulate receptor cells in the cochlea.
• Skin receptors: Receptors that detect touch, pressure, temperature, and pain.
• Tropism: A directional growth response of a plant to a stimulus.
• Phototropism: Growth response of a plant to light; shoots usually grow toward light.
• Auxin: A plant hormone that controls growth; uneven distribution of auxin can cause shoots and roots to bend.

Easily Confused

• Sensory neurone and motor neurone: A sensory neurone carries impulses from a receptor to the central nervous system; a motor neurone carries impulses from the central nervous system to an effector.
• Central nervous system and peripheral nervous system: The central nervous system consists of the brain and spinal cord; the peripheral nervous system consists of all nerves outside them.
• Nerve impulse and neurotransmitter: A nerve impulse is an electrical signal travelling along a neurone; a neurotransmitter is a chemical signal crossing a synapse.
• Reflex action and conscious response: A reflex action is rapid and automatic, whereas a conscious response involves awareness and decision-making.
• Insulin and glucagon: Insulin lowers blood glucose concentration; glucagon raises it.
• Vasodilation and vasoconstriction: Vasodilation widens skin blood vessels and increases heat loss; vasoconstriction narrows them and reduces heat loss.
• Near vision and distant vision: Near vision requires contracted ciliary muscles, slack suspensory ligaments, and a thicker, more curved lens; distant vision requires relaxed ciliary muscles, taut suspensory ligaments, and a thinner, less curved lens.
• Nervous control and hormonal control: Nervous control is fast, short-lived, and localized; hormonal control is slower, longer-lasting, and often more widespread.
• Phototropism and gravitropism: Phototropism is growth in response to light; gravitropism is growth in response to gravity.
• Shoot and root responses to auxin: Auxin promotes elongation in shoots but a higher auxin concentration inhibits elongation in roots.

What Gets Asked

• Describe the general coordination pathway: stimulus → receptor → coordinator → effector → response. Marks are lost by omitting the coordinator or confusing a receptor with an effector.
• Explain a reflex arc: receptor → sensory neurone → relay neurone → motor neurone → effector. Marks are lost by reversing the direction of impulse transmission or omitting the relay neurone.
• Explain accommodation: for near vision, ciliary muscles contract, suspensory ligaments slacken, and the lens becomes thicker and more curved; for distant vision, the opposite changes occur. Marks are lost by interchanging the near- and distant-vision descriptions.
• Explain blood glucose regulation: high glucose causes insulin release and conversion of glucose to glycogen; low glucose causes glucagon release and conversion of glycogen to glucose. Marks are lost by assigning the roles of insulin and glucagon incorrectly.
• Compare nervous and hormonal responses: nervous responses are fast, short-lived, and localized, while hormonal responses are slower, longer-lasting, and often more widespread. Marks are lost by describing hormones as faster than nerve impulses.
• Explain phototropism using auxin: auxin moves to the shaded side of a shoot, where it causes greater cell elongation and bending toward light. Marks are lost by stating that the illuminated side elongates more, or by failing to distinguish shoot and root responses.